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Biofilm Composition and Threshold Concentration for Growth Of PUBLIC AND ENVIRONMENTAL HEALTH MICROBIOLOGY crossm Biofilm Composition and Threshold Concentration for Growth of Legionella pneumophila on Surfaces Exposed to Flowing Warm Tap Water without Disinfectant Downloaded from Dick van der Kooij,a Geo L. Bakker,b Ronald Italiaander,a Harm R. Veenendaal,a Bart A. Wullingsa KWR Watercycle Research Institute, Nieuwegein, the Netherlandsa; Vitens NV, Zwolle, the Netherlandsb ABSTRACT Legionella pneumophila in potable water installations poses a potential Received 1 October 2016 Accepted 13 health risk, but quantitative information about its replication in biofilms in relation http://aem.asm.org/ to water quality is scarce. Therefore, biofilm formation on the surfaces of glass and December 2016 Accepted manuscript posted online 6 chlorinated polyvinyl chloride (CPVC) in contact with tap water at 34 to 39°C was in- January 2017 vestigated under controlled hydraulic conditions in a model system inoculated with Citation van der Kooij D, Bakker GL, biofilm-grown L. pneumophila. The biofilm on glass (average steady-state concen- Italiaander R, Veenendaal HR, Wullings BA. tration, 23 Ϯ 9 pg ATP cmϪ2) exposed to treated aerobic groundwater (0.3 mg C 2017. Biofilm composition and threshold concentration for growth of Legionella Ϫ1 ␮ Ϫ1 liter ;1 g assimilable organic carbon [AOC] liter ) did not support growth of the pneumophila on surfaces exposed to flowing organism, which also disappeared from the biofilm on CPVC (49 Ϯ 9 pg ATP cmϪ2) warm tap water without disinfectant. Appl Ϫ2 Environ Microbiol 83:e02737-16. https:// after initial growth. L. pneumophila attained a level of 4.3 log CFU cm in the bio- on February 16, 2017 by guest doi.org/10.1128/AEM.02737-16. films on glass (1,055 Ϯ 225 pg ATP cmϪ2) and CPVC (2,755 Ϯ 460 pg ATP cmϪ2) ex- Editor Johanna Björkroth, University of Helsinki Ϫ1 ␮ Ϫ1 posed to treated anaerobic groundwater (7.9 mg C liter ;10 g AOC liter ). An Copyright © 2017 American Society for elevated biofilm concentration and growth of L. pneumophila were also observed Microbiology. All Rights Reserved. with tap water from the laboratory. The Betaproteobacteria Piscinibacter and Methylo- Address correspondence to Dick van der Kooij, versatilis and amoeba-resisting Alphaproteobacteria predominated in the clones and [email protected]. isolates retrieved from the biofilms. In the biofilms, the Legionella colony count cor- related significantly with the total cell count (TCC), heterotrophic plate count, ATP concentration, and presence of Vermamoeba vermiformis. This amoeba was rarely de- tected at biofilm concentrations of Ͻ100 pg ATP cmϪ2. A threshold concentration of approximately 50 pg ATP cmϪ2 (TCC ϭ 1 ϫ 106 to 2 ϫ 106 cells cmϪ2) was derived for growth of L. pneumophila in biofilms. IMPORTANCE Legionella pneumophila is the etiologic agent in more than 10,000 cases of Legionnaires’ disease that are reported annually worldwide and in most of the drinking water-associated disease outbreaks reported in the United States. The organism proliferates in biofilms on surfaces exposed to warm water in engineered freshwater installations. An investigation with a test system supplied with different types of warm drinking water without disinfectant under controlled hydraulic condi- tions showed that treated aerobic groundwater (0.3 mg literϪ1 of organic carbon) induced a low biofilm concentration that supported no or very limited growth of L. pneumophila. Elevated biofilm concentrations and L. pneumophila colony counts were observed on surfaces exposed to two types of extensively treated groundwa- ter, containing 1.8 and 7.9 mg C literϪ1 and complying with the microbial water quality criteria during distribution. Control measures in warm tap water installations are therefore essential for preventing growth of L. pneumophila. KEYWORDS Legionella pneumophila, predominating biofilm bacteria, threshold biofilm concentration, warm tap water March 2017 Volume 83 Issue 5 e02737-16 Applied and Environmental Microbiology aem.asm.org 1 van der Kooij et al. Applied and Environmental Microbiology egionella pneumophila is the causative agent of Legionnaires’ disease (LD), a life- Lthreatening pneumonia, and proliferates in natural and engineered freshwater systems at warm temperatures. In 2014, the number of annually reported LD cases amounted to more than 6,000 in Europe (1) and more than 5,000 in the United States (2). The wide range of notifications per million inhabitants (Ͻ1 to 39.4) in European countries indicates that the number of reported cases is an underestimation of the true number of cases. Complicated diagnostics and reporting inefficiency are considered the main reasons for these different notification rates (3). The increased number of reported cases in the United States has been attributed to an increasing population of older persons and persons at high risk for infection and to improved diagnostics and reporting (4). More than two thirds of the cases reported in Europe are community acquired, implying that the source of the infection was not identified, with smaller percentages associated with travel (19%), both abroad and domestic, and with health Downloaded from care (8%) (1). In the United States, LD was the most frequently reported drinking water-associated disease from 2009 to 2012 (5, 6). L. pneumophila is detected only incidentally in distributed drinking water in tem- perate regions, but the organism is commonly present in potable water installations in hotels, hospitals, and residential water systems (1, 6–8). L. pneumophila is a nutritionally fastidious organism requiring specific amino acids for growth (9, 10), but a variety of free-living amoebae grazing on biofilms and sediments can serve as hosts and enable http://aem.asm.org/ its proliferation in the aquatic environment (11–16). The optimal growth temperature of L. pneumophila ranges from 37 to 42°C (17, 18), and a warm temperature is essential for its growth because host amoebae digest the bacterium at temperatures of Ͻ20°C (19, 20). Several studies showed that the colony counts of L. pneumophila in potable water installations correlated with the concentrations of organic carbon, iron, manga- nese, and corrosion products in the water (7, 21–23). However, in a recent study, no growth of culturable L. pneumophila was observed in water contained in glass bottles incubated at 32 to 37°C and supplemented with various amounts of ozonated fulvic on February 16, 2017 by guest acid (24). Colony counts of L. pneumophila in water from systems with iron pipes were higher than those in water from systems with pipes of steel and copper (25). Organic polymeric materials used in water installations, e.g., natural and synthetic rubber, polyethylene (cross-linked), polypropylene, and polybutylene, can also promote growth of L. pneumophila (26–31 ). The L. pneumophila colony counts in biofilms on a variety of materials in contact with tap water in experimental systems correlated significantly with the total cell count (TCC) (31) and the ATP level (30) but not with the heterotrophic plate count (HPC) (29). Water temperature management and prevention of stagnation are essential for limiting the growth of L. pneumophila in potable water installations (32–35). Further- more, the use of plumbing materials that do not promote biofilm formation has been advocated (26–29, 36). However, water quality also affects biofilm formation, but the relationship between the concentration of the water-induced biofilm and growth of Legionella is still unclear. Therefore, a test system, the boiler biofilm monitor (BBM), was developed to determine the effects of drinking water without disinfectant on the biofilm formation and growth of L. pneumophila under optimal temperature and hydraulic conditions resembling those in potable water installations (37). This system was tested in the laboratory by using the locally available drinking water and at two groundwater supplies distributing drinking water with either a very low concentration of natural organic matter (NOM) (Ͻ0.5 mg C literϪ1) or a high NOM concentration (7.8 mg C literϪ1). The abundances and identities of Legionella spp. and free-living protozoa in these unchlorinated supplies were reported earlier (38, 39). The objectives of the present study were to (i) assess the relationship between the biofilm concentration and the Legionella colony count in biofilms under controlled conditions in a model system, (ii) identify bacteria predominating in biofilms, (iii) determine the effect of water quality (NOM and assimilable organic carbon [AOC]) on biofilm formation, and (iv) compare the use of glass and chlorinated polyvinyl chloride (CPVC) in the test system. March 2017 Volume 83 Issue 5 e02737-16 aem.asm.org 2 Threshold Biofilm Concentration for Legionella Growth Applied and Environmental Microbiology TABLE 1 Concentrations of total organic carbon (TOC), assimilable organic carbon (AOC), and ATP in the feed water at the test locationsc Water TOC concn AOC concn (␮g acetate % AOC concn after6hat70°C % ATP concn (supply/locationa (mg liter؊1) C equivalents liter؊1) NOXb (␮g acetate C equivalents liter؊1) NOXb (ng liter؊1 Ϯ Ϯ Ϯ Ϯ AT 0.32 0.01 (4) 1.1 0.3 (3) 75 1.9 0.1 (2) 70 0.7 0.6 (13) Ϯ Ϯ Ϯ AD 0.26 0.03 (3) 1.0 0.1 (2) 94 NA NA 0.9 0.7 (13) Ϯ Ϯ Ϯ Ϯ BT 7.9 0.15 (3) 10.9 1.2 (5) 89 22.9 2.8 (5) 94 10.3 2.6 (29) Ϯ Ϯ Ϯ Ϯ BD 7.9 0.16 (3) 8.9 0.7 (1) 90 19.7 3.3 (1) 97 9.6 3.2 (13) Ϯ Ϯ Ϯ Ϯ CD 1.8 0.1 (2) 3.4 0.4 (3) 77 7.9 2.6 (2) 87 3.0 1.4 (14) a AT, groundwater supply A, treated water (T); AD, supply A at a location in the distribution system; BT, supply B, treated water; BD, supply B at a location in the distribution system; CD, groundwater supply C at a location in the distribution system (D).
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